Apparatus for refining oils or fats

ABSTRACT

Method and apparatus for refining oil which in heated condition is used in the production of consumable products such as French fried potatoes by feeding the oil immediately after starting the baking process through a filter system within the time within which free fat acids or fat acid oxydation products could be formed.

This application is a division of our copending application Ser. No.222,863, filed Jul. 22, 1988.

The invention relates to a method for refining oils or fats which areused in a container in the heated condition for cooking an edibleproduct therein, which refining is carried out by passing the oil or thefat through a filter. In cooking edible products in hot oil or fat, suchas frying or deep-frying potato chips (French fries), croquettes,pancake rolls, etc., the problem arises that the oil becomescontaminated in a relatively short time. This contamination is dependenton the nature of the product processed in the oil and, depending on theproduct, consists of contamination with a varying size range andclogging-up capacity. The burning of said contamination results in thegeneration of smoke and the deposit on the products, while themicroscopically small particles produce the typical frying smell. In thecase of installations which are in operation for prolonged periods thismeans a very unpleasant working atmosphere for the staff, while theproduct to be delivered rapidly becomes bad. Furthermore, as a result ofheating the oil is subject to oxidation which is promoted by the oxygenintroduced along with the product to be deep-fried.

The oxidation and the microscopic product contamination are the cause ofthe formation of fatty acid oxidation products. Furthermore, hydrolysisoccurs due to the introduction of water with the product to bedeep-fried, as a result of which free fatty acids are formed.

A limit is imposed in relation to the fatty acid content by theKeuringsdienst van Waren (Food Inspection Department).

Cleaning the oil with a filter is known. However, this is either notdone at the right time, viz. after the oil has been used, orcontinuously but to an inadequate extend and in a manner which resultsin the oil deteriorating, for example, as a result of the introductionof oxygen. Little is thereby achieved. Although coarse particles can beseparated off in said manner, the very fine particles and the fattyacids or fatty acid oxidation products, or the water, remain present inthe oil.

The object of the invention is therefore to provide an effectivesolution for this problem and to appreciably prolong the service life ofthe oil or the fat content of the container of such an oil, and toremove substances, such as fatty acid oxidation products or free fattyacids, which are injurious to health.

According to the invention this object is achieved in that the oil orfat content of the container is always completely passed through afilter system and back into the container within a certain time afterthe start of the cooking process, which time is determined by the timein which the injurious substances proceed to form free fatty acids orfatty acid oxidation products.

The invention is based on the surprising insight that no free fattyacids or fatty acid oxidation products are formed by oxidation andmicrocontamination, in particular, as a result of removing from the oilin good time injurious substances which are introduced and formed duringfrying, as a result of which the latter are unable to effect the qualityand the stability of the oil or the fat adversely.

According to the invention, for this purpose, the oil or the fat ispassed through a series of filter stages with decreasing pore size andconsequently increasing flow capacity, which is understood to mean thata quantity of oil flowing through a filter stage per unit time can flowtherethrough without any forcing of the flow being carried out. In thecase of edible oils or fats, care has to be exercised sincesaponification occurs if pressures are too high and vortice currents aretoo strong. Increasing fineness of the filter therefore requires alarger surface. The first stage preferably has a pore size which is lessthan 30 μm (micrometer), preferably even less than 25 μm, while thefinal stage still has to separate off 0.1 μm particles.

In order to avoid imposing too much load on the oil, it is alsodesirable, according to the invention, that the oil or the fat is suckedand not pressed through the first filter stage and, in particular,sucked with a vacuum not exceeding 0.5 bar. The suction is of importancesince a pressure drop exceeding 1 bar cannot then occur. With a pressurepump, the pressure will be able to increase excessively if the filterclogs up. As a result of the suction, separation of water, air and oilalso takes place. In the said first filter stage, which is preferablyconstructed in the form of a rotating filter, the separation is thencarried out of the nonmutually adhering coarse granular particles whichare collected in a type of filter known per se to the person skilled inthe art in a tank situated at the bottom of the housing of the filterand can be removed therefrom. Removal of the granular particles meansthat particles which could in fact adhere to each other are able to passthrough the filter and do not give rise as a result of settlement toclogging up of the filter surface or reduction thereof.

In the final stage, the pressure is higher but does not exceed 5 bar.There the very small particles are consequently separated off and here aseparate pressure pump which forces the oil through the very fine filter(microfilter) is preferably used. In said microfilter, the water mayremain behind. The oil passes therethrough along with the air and theair can be separated from the oil during the return to the oven.

Between the first and final stage, one or more filters are used whichhave a pore size not exceeding 5 μm with a pressure not exceeding 3 bar,which filters are connected to the pressure side of the pump which sucksthe oil to be purified out of the oven and through the rotating filter.

The apparatus for carrying out the method according to the inventioncomprises a heatable container which, according to the invention, isconnected to a filter system in which the first stage comprises arotating filter which, via a suction line, is in communication with theliquid content of the container and with a suction pressure pumpdownstream of said filter, a second stage which comprises one or moresurface filters arranged in series upstream of said suction pressurepump and a final stage which comprises a microfilter with a suctionpressure pump in the line between the second stage and the final stage.

In this connection, it may be of benefit that at least the microfilterof the final stage can be heated and this heating can be brought aboutelectrically by heating the container of said microfilter or can beobtained by constructing said container with a double wall and passingthe hot oil which originates from the second stage into said double wallbefore it enters the filter. The electrical heating or heating by meansof a part of the hot oil may be switched on and off with the aid of athermostatic switch placed in said microfilter. Said heating serves toobtain the required viscosity which is necessary to force the oilthrough the microfilter.

The purified oil which emerges from the final stage is fed back to thecontainer in which the deep-frying is carried out. According to theinvention, this preferably takes place via a bell jaw which projectsinto the liquid along with the mouth of the return line above the levelof the liquid and has an air removal pipe. Air which could be present inthe oil can then be liberated and escape.

The entire system may virtually be considered as a closed system becauseoxygen can no longer enter during the filtration.

The various particles which are separated off in the filters are nolonger able to burn because they do not return to the oven.

The contamination introduced into the oil with the products to betreated therefore no longer come into contact with the products as acombustion organism.

As a result of this purification, the oil has a better heat transfercapacity, as a result of which overheating of the oil no longer takesplace when the latter flows past the heating elements.

The invention will now be explained in more detail with reference to thedrawings.

FIG. 1 shows diagrammatically the general design of the apparatus forapplying the method according to the invention,

FIG. 2 shows diagrammatically the first filter stage,

FIG. 3 shows diagrammatically a filter of the second stage, and

FIG. 4 shows diagrammatically the microfilter.

In FIG. 1, 1 indicates a container filled with deep-frying fat, thelevel of which is indicated at 2.

Situated at 3 is a suction bell jar which is connected to the housing 5of a rotating filter via a line 4. The filter unit is indicated by 6 andmay comprise segments rotating with respect to each other and stationarysegments, of which the rotating segments are driven by a motor 7. Dirtparticles are collected on the outside wall of the filter unit 6 anddrop down into the base section 8.

Oil passing through the filter is sucked via the interior of the filterand the line 9 in communication therewith by a pump 10, in particularwith a vacuum of 0.5 bar, which pump 10 forces oil via the line 11 intoa series of surface filters 12 and 13 in which the filter body isdiagrammatically shown by the line 12'. Two surfaces filters 12 and 13are shown, but the number of filters to be used is in this casedependent on the required flow capacity, allowance being made for thepore size of the filter units. The filter cartridges in said filters 12and 13 have to be replaceable. The line connections such as 14 and 15connected at the base have shutoff valves 16 for removing oil and dirtremaining behind in the housings 12 and 13 when cleaning of theinstallation and replacement of the filter cartridges is necessary.

The oil emerging from the last filter unit of the second stage is forcedwith the aid of the pump 10' into the microfilter 17, which may have aheatable wall 18 and is so constructed that it retains particlesmeasuring less than one micron.

Purified oil emerging from said last stage returns via the line 19 tothe vessel 1 and debouches into a bell jar 20 at a point situated abovethe liquid level 2. Said bell jar has an air removal pipe 21.

The rotating filter 5, 6 indicated diagrammatically in FIG. 2 with drivemotor 7 has a removable collection tank 22 in the bottom section and afunnel-shaped guide 23 above it. In said tank dirt particles may becollected which drop off the filter unit 6 and can be removed with thetank. The tank also has a drainage line 24 with stop cock 25.

FIG. 3 shows diagrammatically a filter unit of the second stage. Thefilter 12' may comprise a filter sleeve of non-woven acrylic fibreswound round a support which provides a hollow core so that oil passingthrough the filter sleeve can enter the drainage line 27 via the funnel26. The lines 14 and 15 again have shutoff valves 28 and 29 which arenecessary for cleaning the system. Said filter units have a cover 30 towhich the filter cartridge 12' is attached and by means of whichreplacement is possible.

FIG. 4 shows diagrammatically the microfilter of the last stage. Theinlet line 27 from the last filter unit 13 (FIG. 1) is fed to the doublewall 18.

The oil entering the double wall is able to leave the latter via theoutlet or return line 19. If the pump 10' starts to operate, then itsucks approximately three quarters of the oil supplied and forces thelatter through the lines 33 into the housing of the microfilter andthrough the filter 17. The oil which has passed through the filterreturns via the line 32 to the jacket 18 and proceeds to the discharge19.

In the jacket there is a temperature sensor which switches on the pump10' on reaching the correct temperature, for example 120° C.

The filter cartridge 17, which must be able to retain particlesmeasuring 0.1 micrometer may in this case comprise a piece of fibrematerial, for example also acrylic fibres, wrapped round a support.

With the method and the apparatus according to the invention, anappreciably longer oil or fat service life is achieved, and inparticular a multiple of the usual service life. A finer edible productis obtained which has a better taste and does not have the often bittertaste of products which have been treated with dirty oil. The product istherefore healthier. The smoking (generation of smoke) from the fat nolonger occurs so that the working atmosphere is improved and thedeep-frying smell no longer occurs.

Obviously, the entire installation, including the various filter stages,should be matched to the product to be treated and consequently to thedegree of contamination which may occur, and obviously, in designing thevarious filter stages account should be taken of the liquid contentwhich has to have been completely circulated at least within the time inwhich fatty acids can be formed. Obviously, the various filters shouldbe regularly replaced.

It is important that it is possible for the first time, as a result ofthe method and apparatus according to the invention, to avoid deep-friedfoodstuffs being unhealthy as a result of the previously excessivecontent of fatty acid oxidation products (mutagenic compounds),something against which the Keuringsdienst van Waren (Food InspectionDepartment) and other authorities which are concerned with public healthhave always warned.

This applies also to oxidation products which are growth-inhibiting andare harmful to the functioning of the liver.

What is claimed is:
 1. An apparatus for refining oils or fats comprisinga heatable container and a filter system, said filter system having atleast three stages, a first stage of the filter system comprising arotating filter which, via a suction line, is in communication with theliquid content of the container and a suction pressure pump provideddownstream of the filter, a second stage of the filter system comprisingone or more surface filters arranged in series downstream of the suctionpressure pump, and a third stage of the filter system comprising amicrofilter with a pressure pump in the line between the second stageand the third stage.
 2. Apparatus according to claim 1, characterized inthat at least the microfilter of the third stage can be heated. 3.Apparatus according to claim 1 or 2 further comprising a return lineprovided between said third stage and said container and an airseparator provided along said return line, said air separator comprisinga bell jar which projects into the liquid content of said container, themouth of said return line positioned above the level of the liquid insaid container, and an air escape pipe extending outward from said belljar.